The usual prior art angular rate sensors include a spinning mass to provide a reference direction. However, such sensors have inherent problems which include high drift rates caused by friction and unwanted torques. The ring laser gyroscope, for the most part, overcomes the problems inherent in the prior art sensors. The operation of the ring laser gyroscope is predicated entirely on optical and electronic principles, and angular motion of the ring laser gyroscope is measured by massless light waves circulating in a closed path.
Ring laser gyroscopes are illustrated and described in U.S. Pat. Nos. 3,373,650 and 3,467,472, which issued in the name of Joseph E. Kilpatrick. The ring laser gyroscopes shown and described in the patents include a triangular block which forms a triangular-shaped ring laser cavity defined by three corner mirrors. A triangular-shaped block is preferred since it requires a minimal number of mirrors. The cavity is filled by a gas laser which comprises, for example, helium and neon gas, usually operating at one of two wavelengths; specifically, either at 1.15 micrometers in the infrared spectral band, or at 0.63 micrometers in the visible wavelength region.
Through proper choice of the ratios of the two neon isotopes Ne.sup.20 and Ne.sup.22 in the gas mixture, two monochromatic laser beams are created. The two laser beams respectively travel in clockwise and counterclockwise directions around the triangular cavity in the same closed optical path.
With no angular motion about the input axis of the ring laser gyroscope, the lengths of the two laser beams are equal, and the two optical frequencies are the same. Angular movement of the prior art ring laser gyroscope in either direction about its input axis causes an apparent increase in the cavity length for the beam travelling in the direction of such angular movement and a corresponding decrease for the beam travelling in the opposite direction. Because the closed optical path is a resonant cavity providing sustained oscillation, the wavelength of each beam must also increase or decrease accordingly. Angular movement of the ring laser gyroscope in either direction about its input axis, therefore, causes a frequency differential to occur between the two beam frequencies, and which differential is proportional to the angular rate.
In accordance with the prior art practice, the two beams are extracted from the laser at its output mirror, and they are heterodyned in a beam combiner to produce an interference pattern. The interference pattern is detected by a photodetector which senses the beam frequency of the heterodyned optical frequencies of the two beams, and this beat frequency is a measure of the angular rate.
A difficulty arises in ring laser gyroscopes at low angular rates, in that the frequency differential between the two beams is small at the low rates, and the beams tend to resonate together, or "lock-in" so that the two beams oscillate at only one frequency. It therefore is difficult to read low angular rates because the frequency differential proportional to the angular rate does not exist. A technique is described in U.S. Pat. Nos. 3,373,650 and 3,467,472 for obviating such lock-in at the low angular rates. This technique comprises subjecting the sensing apparatus to a mechanical vibrating effect so that the beams appear to be circulating at a rate higher than the lock-in rate.
Ring laser gyroscopes are sensitive to temperature gradients across their line of symmetry since such gradients affect the Langmuir flow. The Langmuir flow is caused by cataphoretic pumping between the anode and cathode of the laser, and the flow is usually well balanced by careful matching of the capillary bores which contain the glow discharge, and by the utilization of two symmetrically placed glow discharges. The Langmuir flow is also usually balanced in the prior art gyroscopes by maintaining a constant current discharge in the two glow discharges by means of two active regulators.
The prior art ring laser gyroscopes, such as described above, are extremely sensitive to environmental and warm-up temperature changes. Such temperature changes produce temperature gradients across the plane of symmetry of the prior art gyroscopes, this being due to the lack of symmetry of the triangular block. These gradients result in the appearance of spurious output pulses in the absence of any angular movement of the instrument about its input axis. The block of the prior art ring laser gyroscope is purposely made asymmetrical so as to facilitate the prevention of lock-in by the mechanical vibration described above. As explained, lock-in has a tendency to occur at the low input angular rates, as the angular input falls below a certain critical or threshold value, and in a region where a non-linear relationship exists between the input and output of the gyroscope. A substantially linear relationship exists between the input and output of the gyroscope above the lock-in region.
The prior art ring laser gyroscopes are mechanically vibrated at a relatively high frequency in a range, for example, of 100-200 Hz. Residual lock-in effects are evident in such prior art gyroscopes which cause discrete non-linearities between the inputs and outputs thereof. It is common practice in the prior art to use a pseudo-random dither motion in an attempt to reduce such non-linearities.
Similar to the unit described in the copending application, the present invention also provides a gyroscope which includes a block having a triangular-shaped ring laser resonant cavity defined by three corner mirrors, including an output mirror. A gas laser is an integral part of the cavity which provides monochromatic light, and which comprises a capillary glow discharge in a helium and neon gas mixture. A case is provided with a support post. The block is supported on the case by a bearing (not shown) centered at point A on the line of symmetry of the block. A number of springs, all located in the plane of symmetry of the cavity, are provided to support the block at its center on the post. Like the unit in the copending application, no asymmetrical cut-outs, or displaced holes, are provided in the structure of the present invention, so that any change in environmental or warm-up temperature does not produce temperature gradients across the line of symmetry of the instrument.